1. Field of the Invention
The present invention relates to a switched capacitor for use in an over-sampling analog-to-digital converter or similar circuits.
2. Description of the Prior Art
Switched capacitors are widely used, in particular to achieve over-sampling or similar processing, as a device for detecting a change in potential by converting it into amount of electric charge stored therein. In practical use, a switched capacitor is, during a sampling period, charged by being connected to an input potential to be measured, and, during a detection period that follows the sampling period, the switched capacitor is disconnected from the input potential and instead connected to an integrating circuit or a similar circuit in order to detect the charge stored therein.
FIG. 5 shows the construction of a conventional switched capacitor along with an integrating circuit connected thereto. The switched capacitor 90 consists of a single capacitor 95 and four switches 91 to 94. One electrode of the capacitor 95 is connected either to an input potential V.sub.IN or to a reference potential V.sub.REF through the switch 91 or 92, respectively and the other electrode of the capacitor 95 is connected either to the integrating circuit 98 or to the reference potential V.sub.REF through the switch 93 or 94, respectively. The reference potential V.sub.REF to which one electrode of the capacitor 95 is connected through the switch 92 and the reference potential V.sub.REF to which the other electrode of the capacitor 95 is connected through the switch 94 are equal.
The four switches 91 to 94 operate in synchronism. During a sampling period, the switches 91 and 94 are closed, whereas the switches 92 and 93 are open; during a detection period, the switches 92 and 93 are closed, whereas the switches 91 and 94 are open. Accordingly, during the sampling period, the capacitor 95 is charged by the potential difference between the reference potential V.sub.REF and the input potential V.sub.IN, and, during the detecting period, the capacitor 95 adds the potential arising from the charging to the reference potential V.sub.REF and outputs the resultant potential to the integrating circuit 98. Thus, the output potential V.sub.OUT ' of the switched capacitor 90 is expressed as V.sub.OUT '=V.sub.REF -V.sub.IN.
The integrating circuit 98 consists of an amplifier 96 and a capacitor 97. The amplifier 96 receives, at its inverting input terminal, the output potential V.sub.OUT ' of the switched capacitor 90 and, at its non-inverting input terminal, the reference potential V.sub.REF. The capacitor 97 is connected in parallel between the output terminal and the inverting input terminal of the amplifier 96. Thus, the integrating circuit 98 integrates the difference between the output potential V.sub.OUT ' of the switched capacitor 90 and the reference potential V.sub.REF.
During the detection period, the integrating circuit 98 continues integrating the output potential V.sub.OUT ' of the switched capacitor 90 and outputs a potential V.sub.0 ' that corresponds to the amount of charge stored in the switched capacitor 90. Let the capacitance of the capacitor 95 be C' and that of the capacitor 97 be C.sub.NF '. Then, the output potential V.sup.0 ' of the integrating circuit 98 is expressed as V.sub.0 '=C'/C.sub.NF '.multidot.(V.sub.IN -V.sub.REF).
The reference potential V.sub.REF fed to the non-inverting input terminal of the amplifier 96 of the integrating circuit 98 is equal to the reference potential V.sub.REF fed to the switched capacitor 90, both being supplied from a single power supply circuit. This reference potential V.sub.REF is supplied also to other circuits that are not shown in the figure.
This, however, causes a problem: the current that charges the switched capacitor 90 during the sampling period, as well as the current that results from the discharging of the switched capacitor 90 during the detection period, causes variation in the reference potential V.sub.REF. This variation is greater as the capacitance C' of the capacitor 95 is greater, and is harsher as the sampling and detection periods are shorter. Variation in the reference potential V.sub.REF causes unstable operation of not only the integrating circuit 98 but all the circuits that use the reference potential V.sub.REF, and thus leads to reduced reliability of any circuit or device that includes the switched capacitor 90.
To secure stable operation of related circuits, it is essential to prevent the charging and discharging currents from affecting the reference potential V.sub.REF. To achieve this, it has been customary to take some external measures, such as using an unduly high-capacity, high-performance power supply circuit to produce the reference potential V.sub.REF, or using a high-capacitance capacitor to absorb the charging and discharging currents. As a result, with conventional switched capacitors, it has been inevitable to adopt an unduly large-scale circuit design and use an undue number of externally fitted parts, and thus it has been difficult to achieve sufficient simplification of design and reduction of size in circuits and devices that employ such switched capacitors.